linux-kernel - Re: [PATCH] ad7877: keep dma rx buffers in seperate cache lines

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Date:	Thu, 6 May 2010 14:46:04 -0400
From:	Mike Frysinger <vapier.adi@...il.com>
To:	Oskar Schirmer <os@...ix.com>
Cc:	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-input@...r.kernel.org, linux-kernel@...r.kernel.org,
	Daniel Glöckner <dg@...ix.com>,
	Oliver Schneidewind <osw@...ix.com>,
	Johannes Weiner <jw@...ix.com>
Subject: Re: [PATCH] ad7877: keep dma rx buffers in seperate cache lines

On Thu, May 6, 2010 at 06:37, Oskar Schirmer wrote:
>  struct ser_req {
> +       u16                     sample;
> +       char                    __padalign[L1_CACHE_BYTES - sizeof(u16)];
> +
>        u16                     reset;
>        u16                     ref_on;
>        u16                     command;
> -       u16                     sample;
>        struct spi_message      msg;
>        struct spi_transfer     xfer[6];
>  };

are you sure this is necessary ?  ser_req is only ever used with
spi_sync() and it's allocated/released on the fly, so how could
anything be reading that memory between the start of the transmission
and the return to adi7877 ?

>  struct ad7877 {
> +       u16                     conversion_data[AD7877_NR_SENSE];
> +       char                    __padalign[L1_CACHE_BYTES
> +                                       - AD7877_NR_SENSE * sizeof(u16)];
> +
>        struct input_dev        *input;
>        char                    phys[32];
>
> @@ -182,8 +188,6 @@ struct ad7877 {
>        u8                      averaging;
>        u8                      pen_down_acc_interval;
>
> -       u16                     conversion_data[AD7877_NR_SENSE];
> -
>        struct spi_transfer     xfer[AD7877_NR_SENSE + 2];
>        struct spi_message      msg;

i can see the spi_message inside of this struct being a problem
because the spi transfer is doing asynchronously with spi_async().
however, i would add a comment right above these two fields with a
short explanation as to why they're at the start and why the pad
exists so someone down the line doesnt move it.
-mike